Building a digital scale from scratch

[Raivis] was given a particular task at his university – find a way to measure how many Duplo bricks are stacked together. There are a number of ways to do this, everything from computer vision to using a ruler, but [Raivis] chose a much more educational method. He built a digital scale from scratch out of a strain gauge and a Wheatstone bridge. The build log is immensely educational and provides some insight into the challenges of weighing things digitally.

A strain gauge is a simple piece of equipment, just a small force sensitive resistor. When attached to a metal bar and a force is applied, the resistance inside the strain gauge changes, but not by much. There’s only a few micro Ohms difference between the minimum and maximum of [Raivis]’ load cell, so he needed a way to measure very slight changes in resistance.

The solution was a Wheatstone bridge, or four resistors arranged in a square. When one of the resistors in the bridge is replaced with a strain gauge, very small changes in resistance can be measured.

With a custom ‘duino amplifier shield, [Raivis] can measure the resistance of his load cell with 10-bit resolution, or a maximum weight of 1.32 kg with a resolution of two and a half grams. A single duplo block weighs about 12 grams, so we’ll call this one a success.

The real gem of this design is the (eventual) correct fabrication of the load cell. The Author points to this link (which is very good, and therefore worth repeating here) which explains this in detail:

Can someone explain in a little detail how to connect a piece of metal/wood to the load sensor (https://www.sparkfun.com/products/10245) How do you ensure that all the pressure from whatever platform you build is going directly on the sensor?

He states that “superglue doesn’t work”… this is wrong. I’ve used superglue many times, it’s on my blog keithhack.blogspot.ca and in my last and current job — the difference is in surface preparation, using an accelerator on the gages, and you apply they in a specific way. Vishay has a tech note on it…. their cyanoacrylate is called m-bond 200, it’s “superglue” just a highly tested consistent quality glue. I’ve used it on hundreds of strain gages for 4 years now. Epoxy is the better long term solution, but cheap off the shelf epoxies have their own issues — don’t use one with filler!